This is a sharp answer with some good humor to a lot of unnecessary nit
picking that goes on, on Tower Talk. They never ever contribute any useful
info themselves--it's too risky. Any comments. K7GCO
In a message dated 6/7/01 8:36:44 PM Pacific Daylight Time,
k2av@contesting.com writes:
<< Ken blends a number of things together in his post. Let's separate them
out:
(1) Proportions are at issue. Most of the time I have seen a tribander and
a
two meter 10 element or some such up together, the two meter job has
been five, six, ten feet above the tribander. This is like 50 or 100
feet separation on HF. The elements near the 2m driven element are
something like ten, twelve inches apart. The interactions between the
2m elements have the time (space) to form before the tribander below
comes into play. To keep it analogous or proportional with the HF
problem, IF you put that 2m beam inline with the tribander boom, only
five inches above it, you WOULD get notable interaction, because one of
the elements of the tribander is JUST AS TIGHTLY COUPLED into the 2m
beam as its other elements.
(2) In the case of an 8JK, both elements are driven and create PRIMARY
areas
of partial cancellation IN THE NEAR FIELD. None such happens with a
yagi. The primary radiation from the driven element is like a dipole.
Only hole is off the ends.
(3) In the case of an HF triband or monoband yagi (what most people on this
reflector are talking about) at the top of a tower with guy wires to
within a few feet of top, the guy wire is right smack in the dense part
of the near field, CAN have significant current on it, CAN distort the
pattern. Put it in the model. It doesn't radiate only when it
serendipitously just happens to balance out by some happy aspect of the
instant orientation of the yagi elements and the length and orientation
of the guy wire. From there rotate the yagi a bit and you're back into
interaction. Or don't rotate it and see how much is radiating off the
other two guys.
(4) On HF that upper guy is just like a snake in your pant leg. Plenty close
enough to cause trouble. Best not ignore it.
73
>>
Guy blends a number of things together in his post. Lets separate them.
In regard to your 1st paragraph this example is "not comparable to guy wires
45 degrees from a tower" -- the "main example" of my short post. Answers
have to be relatively short and all extreme examples as you suggest can't be
covered. These posts are not a Doctors Thesis as some of you seem to think.
The typical TT Post answers are not complete comprehensive articles where
many typical and extreme examples are given. Logical assumptions and
exceptions are expected to be made by the readers and even by all the Know It
Alls. Patterns or pictures can't even be shown on TT. The reader is
expected to run his own patterns in Eznec and do some actual pattern
measurements. Nit picking without the courtesy of at least previous
correspondence is really not ethically justified with all these technical
limitations.
Further a 5" spacing between the 2M beam and the tri-bander boom as you
suggest above is "totally absurd". There is nothing "analogous or
proportional with the HF" (your scaling is way off) in this comparison and
I'd seriously question large coupling of the 2M beam to the tribander down
the boom that the tribander sees. I would expect some coupling of the
tribander to the 2M beams free space pattern. Only a very misinformed Novice
would space or even suggest a 2M beam that close to a tribander! So why even
bring it up? At least be fair in your nit picking. 5' not 5" spacing is
frequently used but I use 10' for 2M beams over other beams. This winter I
used a 10' spacing for a 10M 3 element beam tuned for max gain over a 5
element 6M beam. The 6M beam had no affect on the 10M beams SWR or
performance--in Eznec either. Since you suggest this as a flaw in my
suggestions, you show me patterns of a tribander beam in free space on all 3
bands and then 5" under a 2M 11 element beam in free space for affect just to
"actually support" your contention and not your opinion. I test 2M beams in
my basement for SWR and right below water pipes that don't up set the SWR
even up close. There may be some affect to the free space pattern but I or
anyone would have to put it in Eznec to see what affect they have and the
pipes are too many WL long to duplicate a tribander boom. I found that if
the SWR was good on a commercial 2M beam in my basement it was good on the
tower with less surrounding objects. SWR affect is all I can test in my
basement. Try again Guy--you struck out with a very bad example. What was
your real goal here that served a useful purpose?
One of my test frequencies for scale models of quads, yagi's and wire
antennas is the 100 MHz BC FM band. These are constant RF sources 24 hours a
day. Although I have a JFD FS meter, I often use my FM receiver with
S-meter. Some still have horizontally polarized transmitting antennas.
Vertically polarized 144&450 MHz repeater antennas are very difficult to test
with as in this case the support is the same polarization as the antenna.
There are ways to do it but it's a mess and time consuming. TV stations can
be used if horizontally polarized.
Now I have run a couple thousand patterns on pattern recorders and many
more on the air checking all the specific factors of "Gain, F/B and Nulls".
How many have you run--on paper where you checked the Eznec or mfgs pattern
on your tower with and without guys for example? Show me the patterns. I've
developed very good insights into what upsets free space patterns and SWR
that continues to check out on the ham bands. It's not possible to properly
cover all this in a few paragraphs and it's about all most TTer's can handle.
Suggestions are really somewhat limited to "General Guide Lines" for obvious
reasons. If you want more info, derive it yourself or ask for more details
in the proper manner instead of nit picking with poor examples. You can dig
yourself some embarrassing holes that way.
In regard to paragraph 2, I will state one more time, make a 2M W8JK and add
directors and reflectors on each end, top and bottom to it and see what
happens in actual practice and in Eznec. They are even much stronger in
their coupling influence than any guy wire not of the same polarization way
out of the main field. I can also assure you I know exactly what the pattern
of a dipole is. When 2 dipoles are close spaced at .125 WL and fed with 180
phasing, things happen you will have to learn about. It was only given as an
example of a "tight vertical pattern" with vertical nulls right over the
tower that everyone else assumed and realized it wasn't exactly similar to a
yagi.
The Raibeam has a slightly closer spacing but a different phasing he
learned to make maximum use of in a beneficial way. Very few have.
I have only said "the higher the gain of the beam it becomes more isolated
from surrounding objects (sees less of them)--not totally isolated". It
depends what you can live with or have to live with. Rotating the beam just
shows the average ham the E plane and the affects on the H plane are seldom
ever known unless actually measured somehow or plotted in Exnec. The W8JK
just also happens to have it's pattern null directly underneath (and above)
where the tower is and the guys start--if there. That's an ideal situation
not shared by yagi's. I thought you would recognize that. I didn't ignore
or go into near field concepts as it was not justified.
Further some near field concepts have been shot down recently. If you can't
live without your near field bit, get a Palomar RF Current Meter and actually
measure the current in each close metal objects to your hearts content or
read it from the Eznec current list. These posts were for the average ham on
TT. I often encourage them to run some tests also on a 2M beam with 10W and
a scaled tower and guys. Use a 2M dipole with a light bulb in the center.
I've done this very thing for talks I had demonstrating pattern immunity of
yagi's around it and how to stop RF Spill Over below a vertical like the AEA
Isopole does although also just using a single radial 1/4 WL below the
regular radials. I do have some solid foundations over all these years for
my many suggestions. If you follow my suggestions you will find much the
same thing. Heaven forbid I'd never lead you astray! I'd never hear the end
of it.
In regard to paragraph 3 I know very well what most hams use as I have used
the same beams. I've run all kinds of patterns and SWR with and without
baluns, with RF ammeters in the DE feedpoint while rotating with and without
guy--on and on--and published data 25 years ago. I specifically recommended
Phillistran guys for a 1/2 wave before a metal guy and also running the 1st
metal guy 10 or more feet below the tower (you have a weak tower if the guys
have to be at the top) with an insulator at the tower and at 5, 15, (also
20') and 30' "from the tower"--so as to help isolate the guys from the beam
and make you feel good. I have found it works great watching the RF ammeters
in the feedpoint, the SWR bridge when rotating and the plotting patterns.
Why do you have a problem with this? I have not ignored anything. I covered
the factors I found important over the years. Run the live RF tests
yourself, then in Eznec and then comment if you feel necessary.
I'll say again--attaching a guy wire at the top of a tower is "totally
absurd", I've never ever done it as it's just "asking for interaction" if not
broken up. If you can't attach guy wires at least 10' below the top of the
tower, your tower is too weak and eliminates the independent use of a LF
Inverted Vee which I have considerable experience with under beams. I first
used it in 1936 and had the first article on it on 8/60 QST. "I've been
there and done that many times addressing this interference problem."
Sometimes slopers have a better match 10-15' below the beam and even at half
the tower length or even lower. I suggested that even in the late 30's and
many times since--it seemed very obvious to a very young student of antennas.
There was no affect observable on the W8JK's or the Mims Beams back then and
they had a broad vertical pattern. A very good investment is a self
supporting tower and/or a telephone pole. Guy wire problems and their
expenses just go away. Their cost could be put towards the cost of a self
supporting tower. What kind of a price savings can you put on not having guy
wire nuisance and interference when up close and unbroken?
In regard to paragraph 4, I never let resonant snakes get into my pant
legs. They could change my life style. If it got close and in my "Near
Field" I'd shoot holes in it with my 45 starting at 1" behind the head, then
5", 10", 15", 20", 30" until I was sure it was "Non-Resonant, Not-Reactive and
with No RF Bite ." I like to keep my Driven Element "Self-Resonant."
Next time I'll try and cover all the combinations anyone can dream up and
perhaps it will eliminate the constant need to clarify examples that are "not
applicable." So far all this info has been FREE of charge. For a
Professional Fee I'll be glad to derive all the data you think I left out and
you are free to nit pick that all day long. Legitimate questions are always
welcome from TTer's as they sometimes create new slants or better ways to do
things and I'm always looking for them. TT should be called "The TT Nit
Pickers Shooting Gallery."
"One Test and Common Sense Is Still Worth 1000 Unnecessary Opinions." (Old
K7GCO Axiom)
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